The present invention relates to a load-controlling electronic device for controlling a current which flows through an electrical load, an igniting unit for an internal combustion engine for generating a high voltage and applying it to an ignition plug, an electronic distributor for an internal combustion engine for distributing a high voltage to an ignition plug of each cylinder, and an ignition timing control unit for an internal combustion engine for controlling the timing of an ignition to a mixed gas to be supplied to an internal combustion engine.
Recently, there have been an increasing number of devices according to which an ignition coil is disposed in each of a plurality of cylinders of an internal combustion engine and a switching element such as a power transistor or the like is disposed corresponding to each of the ignition coils. With this arrangement, it is possible to distribute a high voltage generated by an ignition coil to a specific cylinder by conducting and breaking a current to a specified switching element among a plurality of switching elements.
The connection of individual switching elements to separate ignition coils is done by separate lead frames. In other words, lead frames (lead frames at the ignition coils) of the number, at least the same as the number, of the switching elements are used to electrically connect the ignition coils with the switching elements. Further, the switching elements are connected to the common earth. In this case, the respective switching elements are connected to a common lead frame (a lead frame at the earth side) and then are connected to the earth. The above technique is disclosed in the JP-A-H 1-259550, for example.
A switching element for a transistor or the like is destroyed when an excess current is flown to the switching element. Therefore, a current limiting circuit is provided to limit the current flown to the switching element. The current limiting circuit detects a current which has flown to the switching element and limits the current when a current level of the current flowing to the switching element exceeds a predetermined level.
The current limiting circuit generally detects a current flowing to the switching element by a current detecting resistor. The current detecting resistor occupies a large area in a circuit substrate, and therefore, the current detecting resistor is used commonly. Under this system, a common current detecting resistor is connected to a plurality of switching elements. When a current flowing through this common current resistor exceeds a predetermined level, the current flowing to all the switching elements is limited simultaneously.
When one of the switching elements has been sticked due to an occurrence of a certain abnormal condition and this switching element has been fixed to a conductive state, an excess current flows to the lead frame at the ignition coil side and the lead frame at the earth side. According to the prior-art technique, one end of each switching element is connected to the ignition coil by the lead frame at the ignition coil provided corresponding to each switching element. The other end of each switching element is collected together with the other end of the rest of the switching elements, and they are earthed by a common lead frame at the earth. When the lead frame at the earth side has been sticked by an excess current, supply of a current to all the ignition coils is stopped. In this case, a supply of a current is stopped not only to the ignition coil connected to the abnormal switching element but also to other ignition coils connected to the switching elements which operate normally.